The present invention is broadly directed to an improved apparatus and method for rapidly freezing a plurality of tissue specimens at cryogenic temperatures that enhances heat transfer, quickly cools tissue holders and tissue, facilitates cutting of thin tissue sections and facilitates tracking of tissue specimens throughout a tissue preparation and examination process. More particularly, the invention is directed to a multi-specimen tissue freezing apparatus and method of mapping and labeling, the apparatus including a channel system permitting highly effective circulation of a cryogen and transfer of heat thereto so as to rapidly cool tissue specimens.
Biopsy or surgical removal of tissue specimens for histologic examination is commonly employed for diagnostic purposes. When a lesion is known or suspected to be malignant, the entire mass is generally excised, if possible. An examination technique may be employed in which the entire tumor margin surface area is reviewed under a microscope. This technique involves microscopic screening of the exterior surface area of the tumor for the presence of malignant cells in order to ensure that all such cells have been removed. If practiced effectively, tumor margin surface area examination enhances the likelihood of complete removal of all cells of a localized malignancy.
Once harvested, the tissue sample is preferably quickly frozen at a controlled rate using a cryogenic coolant in order to obtain high quality frozen sections suitable for use in diagnosis. The tissue is then cut into thin layers or sections for histological examination. It is important that the tissue be frozen and the histologic examination performed as quickly as possible, since the patient must be kept waiting pending the microscopic evaluation, in case any additional tissue must be excised. In the past each review of the tissue was comparatively lengthy, so that a patient had to be maintained in a very uncomfortable state with an open wound for a long period of time. Much of the delay was due to slow freezing of the tissue samples, so fast freezing is very desirable, especially where multiple samples must be taken.
Controlled freezing of the tissue may be accomplished using the methods and devices set forth in Applicant's previous patents, such as U.S. Pat. Nos. 4,695,339; 4,752,347; 5,628,197; 5,829,256; 6,094,923, 6,289,682 and 6,725,673, which are incorporated herein by reference. The rate at which specimens can be frozen under such controlled conditions is determined by the rate of heat transfer from a cryogenic fluid, such as liquid nitrogen, to the platform on which the tissue is placed. Specimens must be frozen relatively quickly in order to avoid formation of large ice crystals. However, attempts to increase the rate of freezing by use of excessive amounts of cryogenic material may impair control over the freezing process. Specimens that are frozen unevenly or incorrectly may be marred by voids and artifacts that might impair histologic examination and diagnosis. It is also desirable to minimize the quantity of cryogenic fluid that is used, since such fluids are costly and may present certain environmental hazards which must be addressed. Therefore, it is important to enhance heat transfer while maintaining control over specimen freezing conditions and conserving use of cryogenic fluids.
Even a properly prepared tissue specimen that is quickly frozen under controlled conditions may not result in a high quality histologic specimen unless thin tissue sections can be taken easily from the frozen specimen. Compression of the section may occur where difficulty is encountered in cutting thin sections from a frozen specimen. Upon gross examination, compressed tissue sections may appear to be usable for mounting on slides, but will prove to be difficult to evaluate. Badly crumpled sections may be unusable.
Another problem faced in a busy laboratory is the proper identification of tissue samples as they travel from station to station, through the processes of tissue harvesting; preparations, such as relaxing and anatomic color marking before freezing; freezing which may include more than one step or transfer to and from freezing platforms; slicing; and examination. A further challenge is developing improved methods of mapping or marking both the patient and the tissue sample to ensure correct orientation of the tissue sample with respect to the patient if clinical reorientation is necessary for further tissue harvesting. Accordingly, there is a need for apparatus and methods for evenly and quickly freezing multiple tissue specimens under controlled conditions with correct labeling and mapping.